Optics - Important Formulas, Definitions, and Examples | 11th Physics

Chapter 9: Optics

Formulas: Reflection, Refraction, Snell's Law, Mirror and Lens Formula, Magnification

Introduction

Optics is the branch of physics that deals with the study of light, its behavior, and its interaction with different materials. This chapter covers reflection, refraction, and the applications of mirrors and lenses.

1. Nature of Light

Definition:

Light is an electromagnetic wave that travels in a straight line and can exhibit wave-like and particle-like properties (wave-particle duality).

• The speed of light in a vacuum is approximately 3 x 10⁸ m/s.
• Light travels slower in mediums like water, glass, and other transparent materials compared to a vacuum.

2. Reflection of Light

Definition:

Reflection is the phenomenon where light bounces off a surface. The angle of incidence is equal to the angle of reflection.

• Law of Reflection: ∠i = ∠r (angle of incidence = angle of reflection).
• Reflection can occur on smooth surfaces (specular reflection) or rough surfaces (diffuse reflection).

3. Refraction of Light

Definition:

Refraction is the bending of light as it passes from one medium to another due to a change in speed.

• Snell's Law: n1 × sin θ1 = n2 × sin θ2, where n is the refractive index of the medium and θ is the angle of incidence/refraction.
• Refractive index determines how much light will bend when entering a medium.

4. Lenses and Lens Formula

Definition:

Lenses are transparent optical elements that refract light to form images. There are two main types of lenses: convex (converging) and concave (diverging).

• Lens Formula: 1/f = 1/v - 1/u, where f is the focal length, v is the image distance, and u is the object distance.
• Convex lenses bring light to a focus, forming real or virtual images, while concave lenses diverge light and create virtual images.

5. Mirror Formula and Magnification

Definition:

Mirrors reflect light and form images. There are two main types of mirrors: concave (converging) and convex (diverging).

• Mirror Formula: 1/f = 1/v + 1/u, similar to the lens formula.
• Magnification (m): m = -v/u (for mirrors) or m = h'/h, where h is the object height and h' is the image height.
• Concave mirrors can form both real and virtual images, while convex mirrors only produce virtual, diminished images.

6. Total Internal Reflection (TIR)

Definition:

Total internal reflection occurs when light passes from a denser to a rarer medium and the angle of incidence exceeds the critical angle.

• Critical Angle (C): The angle of incidence for which the angle of refraction is 90°.
• TIR is the basis for fiber optics, used in communication and medical equipment.

7. Optical Instruments

Examples:

Various optical instruments use lenses and mirrors to magnify or modify images:

• Microscope: Uses convex lenses to magnify small objects.
• Telescope: Uses lenses or mirrors to observe distant objects.
• Camera: Utilizes lenses to focus light and capture images.